1. Field of the Invention
The present invention relates to a light receiving element for use in writing data to, and reading data from, an optical disk, such as CD-R/RW, DVD-RAM, or the like. The present invention also relates to a light detector comprising the light receiving element and built-in circuitry, and an optical pickup comprising the light detector.
2. Description of the Related Art
Photodiodes (PDs), which are light receiving elements, are semiconductor devices which convert incident light through their light receiving surfaces to an electrical signal. The PDs are used for optical pickups of optical disk apparatuses which write data onto an optical disk, such as CD-R/RW, DVD-R/RW, DVD-RAM, or the like.
FIG. 12 is a schematic diagram showing a configuration of an optical pickup comprising a photodiode. This optical pickup comprises a semiconductor laser (laser diode) LD for irradiating a predetermined portion of the information recording side of an optical disk 34 (e.g., CD-R/RW) with light for writing data, and a photodiode PD for converting light reflected from the predetermined portion of the information recording side of the optical disk 34 to an electrical signal. The photodiode PD has five light detecting portions D1 to D5.
Laser light emitted from the semiconductor laser LD is split by a tracking beam generating diffraction grating 30 provided between the semiconductor laser LD and a hologram element 31 into three laser beams, i.e., two subbeams for tracking and a main beam for reading information signals. These laser beams are transmitted through the hologram element 31 as zero-order light, and are made parallel by a collimating lens 32, and are condensed by an object lens 33 to form a beam spot on the information recording side of the optical disk 34. Digital information (bit information) is recorded on the tracks of the information recording side of the optical disk 34 by means of formation of pits and lands, magnetic modulation, refractive index modulation, or the like.
The laser beam condensed onto a track of the information recording side of the optical disk 34 is modulated by pits, which are arranged in accordance with information recorded on the track, and is then reflected from the information recording side of the optical disk 34. The reflected light from the information recording side of the optical disk 34 is transmitted through the object lens 33 and the collimating lens 32, diffracted by the hologram element 31, and directed onto the light detecting portion D1 to D5 of the photodiode PD as first-order diffracted light.
The hologram element 31 has two regions with different diffraction pitches, i.e., regions 31a and 31b. A portion of the reflected main beam for reading information signals, which is directed onto one of the two regions of the hologram element 31, is condensed onto a dividing line, which separates the light detecting portions D2 and D3 of the photodiode PD. The other portion of the reflected main beam is directed onto the other region of the hologram element 31, and is condensed onto the light detecting portion D4 of the photodiode PD. The reflected subbeams for tracking are condensed via the hologram element 31 onto the respective light detecting portions D1 and D5 of the photodiode PD.
In the optical pickup of FIG. 12, the position of the photodiode PD onto which the reflected main beam for reading information signals is condensed, is moved in a direction traversing the light detecting portions D2 and D3 depending on the distance between the hologram element 31 and the optical disk 34. When the main beam for reading information signals is brought into focus on the information recording side of the optical disk 34, the reflected main beam is directed onto the dividing line between the light detecting portions D2 and D3 of the photodiode PD.
The outputs of the light detecting portions D1 to D5 are represented by S1 to S5, respectively. A focusing error signal FES is given by:FES=S2−S3. 
Tracking error is detected by condensing the two subbeams for tracking onto the respective light detecting portions D1 and D5 of the photodiode PD, and obtaining a tracking error signal TES. The tracking error signal TES is given by:TES=S1−S5. 
When the tracking error signal TES is 0, the main beam for reading information signals is condensed onto a target track on the information recording side of the optical disk 34.
A reproduction (read out) signal RF for reading data on the information recording side of the optical disk 34 is given as the sum of the outputs of the light detecting portions D2 to D4 of the photodiode PD receiving the reflected main beam for reading information signals, i.e.,RF=S2+S3+S4. 
In this manner, the photodiode PD of the optical pickup detects the reproduction signal RF of reflected light containing a data signal recorded on the information recording side of the optical disk 34. The photodiode PD also detects the focusing error signal FES, which is a focusing signal for adjusting the focus of laser light from the semiconductor laser LD, and the tracking error signal TES, which is an address signal for verifying a position on the information recording side of the optical disk 34 which is irradiated with the laser light. With the focusing error signal FES and the tracking error signal TES detected by the photodiode PD, the optical pickup is controlled so that laser light emitted by the semiconductor laser LD toward the optical disk 34, in which data is recorded, is accurately directed onto a predetermined position on the information recording side of the optical disk 34.
Recently, an optical disk apparatus employing such an optical pickup has been used to write or read a huge amount of data (e.g., video data) to or from an optical disk. As the data amount of an optical disk is increased, there is an increasing demand for a reduction in time required to write data to, or read data from, the optical disk. To meet the demand, a rate at which data is written to or read from the information recording side needs to be increased (e.g., 16× speed, 32× speed, etc.).
Optical pickups for an optical disk apparatus adopt a light detector in which a light receiving element (e.g., the photodiode PD shown in FIG. 12) is integrated with a signal processing circuit on the same device (a light detector with built-in circuitry). A high-speed operation of a photodiode is required for increasing the data write/read rate to the information recording side of an optical disk. To meet the requirement, it is important to improve the response speed of the photodiode to incident light.
The response speed of the photodiode is determined mainly by a CR time constant defined by the junction capacitance (C) of the depletion layer in a PN junction region and the series resistance (R) of a cathode region, an anode region, and the like, and the movement time required for photocarriers generated in a semiconductor layer deeper than the depletion layer to be diffused from the generation region to an end of the depletion layer due to the difference in the photocarrier concentration. The smaller the CR time constant and the shorter the movement time of photocarriers, the faster the response speed of the photodiode.
In the optical disk apparatus, the writing of data to the information recording side of an optical disk is carried out by changing the shape or phase of a pigment on the information recording side of the optical disk with the heat of laser light from the semiconductor laser LD. Therefore, in order to reduce the write time of data to the information recording side of an optical disk, the light power of laser light emitted from the semiconductor laser LD to the information recording side of the optical disk needs to be increased so as to enhance the amount of light. In this case, the amount of light (reflected light), which is reflected from the information recording side of the optical disk and enters the photodiode PD, is increased. As the amount of the reflected light entering the photodiode PD is increased, the photocarriers generated within the semiconductor layer constituting the photodiode are accumulated in the vicinity of the PN junction region. The accumulation of the photocarriers narrows the width of the depletion layer, leading to a phenomenon that the junction capacitance increases. As a result, the cut-off frequency of the photodiode PD is lowered, and therefore the response speed of the photodiode PD may be reduced.
As to the reduction in the response speed of the photodiode PD due to the photocarrier accumulation, if the width of the depletion layer in the PN junction region of the photodiode PD is limited and the intensity of the electric field within the depletion layer is increased, the width of the depletion layer can be prevented from being narrowed due to the accumulation of the photocarriers generated within the semiconductor layer in the vicinity of the PN junction region.
However, if the width of the depletion layer in the PN junction region of the photodiode PD is limited, the photocarriers generated within the semiconductor layer deeper than the depletion layer, due to light entering the light receiving surface of the photodiode PD, are gradually moved from the semiconductor layer deeper than the depletion layer to an end of the depletion layer by diffusion due to the difference in the photocarrier concentration, so that the movement time of the photocarrier is elongated. The increase in the photocarrier movement time may reduce the response speed of the photodiode.
A solution to such a problem is disclosed in Japanese Laid-Open Publication No. 2001-77401 (Publication 1).
FIG. 13A is a cross-sectional view showing a photodiode disclosed in Publication 1. FIG. 13B is a graph showing the impurity concentration distribution of a cross section of the photodiode taken along line W-W′ in FIG. 13A.
The photodiode shown in FIG. 13A comprises a low resistivity P-type semiconductor substrate 7, and a P+-type buried diffusion layer 2 having a higher impurity concentration than that of the substrate 7, a high resistivity P-type epitaxial layer 3 having a lower impurity concentration than that of the substrate 7, and an N-type epitaxial layer 4, which are laminated in this order on the substrate 7. Thus, the photodiode has a multilayer structure. At an interface between the high resistivity P-type epitaxial layer 3 and the N-type epitaxial layer 4, a plurality of P+-type separation buried diffusion layers 12 are provided in predetermined regions. A P+-type separation buried diffusion layer 11 is provided on the P+-type separation buried diffusion layer 12 in the N-type epitaxial layer 4. A surface of the P+-type separation buried diffusion layer 11 is exposed from a surface of the N-type epitaxial layer 4. Thus, in FIG. 13A, the photodiode is provided in a region surrounded by the P+-type separation buried diffusion layer 12 and the P+-type separation buried diffusion layer 11 on the high resistivity P-type epitaxial layer 3.
The impurity concentration distribution of the photodiode in FIG. 13A from the surface to the inside is shown in FIG. 13B.
The impurity concentration of the N-type epitaxial layer 4 is designed to be distributed uniformly from the surface of the photodiode to the PN junction region at the interface between the N-type epitaxial layer 4 and the high resistivity P-type epitaxial layer 3.
In the PN junction region at the interface between the N-type epitaxial layer 4 and the high resistivity P-type epitaxial layer 3, the impurity concentrations of the N-type epitaxial layer 4 and the high resistivity P-type epitaxial layer 3 are compensated by each other and both concentrations are steeply reduced. Therefore, in the PN junction region, a strong electric field is generated, which is directed toward the junction interface from the N-type epitaxial layer 4 and the high resistivity P-type epitaxial layer 3.
The impurity concentration of the high resistivity P-type epitaxial layer 3 is designed to be uniformly distributed and to be lower than the impurity concentration of the N-type epitaxial layer 4 except for the PN junction region.
The impurity concentration of the P+-type buried diffusion layer 2 is designed to be higher than the impurity concentration of the high resistivity P-type epitaxial layer 3, and to have a curved profile with a peak of the impurity concentration. Therefore, a portion of the P+-type buried diffusion layer 2 between a region having the peak of the impurity concentration and the high resistivity P-type epitaxial layer 3, has a higher potential than that of the high resistivity P-type epitaxial layer 3. As a result, an internal electric field is generated in the P+-type buried diffusion layer 2 from the region having the peak of the impurity concentration toward the high resistivity P-type epitaxial layer 3.
The impurity concentration of the low resistivity P-type semiconductor substrate 7 is designed to be uniformly distributed and to be lower than the peak of the impurity concentration of the P+-type buried diffusion layer 2. Therefore, the potential of the region of the P+-type buried diffusion layer 2 having the peak of the impurity concentration, functions as a potential barrier against electrons in the low resistivity P-type semiconductor substrate 7.
In the thus-constructed photodiode, a reduced proportion of the photocarriers, which are generated within the low resistivity P-type semiconductor substrate 7 deeper than the P+-type buried diffusion layer 2, due to light entering the light receiving surface of the photodiode (i.e., the surface of the N-type epitaxial layer 4), can go beyond the P+-type buried diffusion layer 2 and reach the PN junction region at the interface between the high resistivity P-type epitaxial layer 3 and the N-type epitaxial layer 4. This is because the potential of a portion of the P+-type buried diffusion layer 2 nearer the low resistivity P-type semiconductor substrate 7 functions as a potential barrier against the potential of the low resistivity P-type semiconductor substrate 7. Therefore, a large amount of photocarriers generated in the low resistivity P-type semiconductor substrate 7 cannot reach the PN junction region due to the potential barrier of the P+-type buried diffusion layer 2, and are subjected to recombination in the low resistance low resistivity P-type semiconductor substrate 7 to be extinguished.
Accordingly, in the photodiode shown in FIGS. 13A and 13B, the number of photocarriers, which are generated in a deep site within the photodiode, and are moved over a long distance up to the end of the depletion layer (i.e., the PN junction region formed at the interface between the high resistivity P-type epitaxial layer 3 and the N-type epitaxial layer 4) caused by diffusion due to the difference in the photocarrier concentration, can be reduced. As a result, a reduction in the response speed of the photodiode can be prevented.
If the difference in the impurity concentration between the high resistivity P-type epitaxial layer 3 and the P+-type buried diffusion layer 2 is increased as shown in FIG. 13B, the intensity of the internal electric field generated from the P+-type buried diffusion layer 2 toward the high resistivity P-type epitaxial layer 3 is increased. Therefore, the mobility of the photocarrier generated between a region below the depletion layer at the interface between the high resistivity P-type epitaxial layer 3 and the N-type epitaxial layer 4, and a region nearer the surface of the photodiode than the region having the peak of the impurity concentration of the P+-type buried diffusion layer 2, is increased. Therefore, the response speed of the photodiode can be improved, even when the information recording side of an optical disk is irradiated with a large amount of light in order to reduce the write time of data to the information recording side of the optical disk, so that a large amount of reflected light enters the photodiode.
Further, even when the information recording side of an optical disk is irradiated with a large amount of light in order to reduce the write time of data to the information recording side of the optical disk, so that a large amount of reflected light enters the photodiode, the thickness of the high resistivity P-type epitaxial layer 3 may be reduced by a predetermined value, and the width of the depletion layer may be limited so as to increase the intensity of the electric field within the depletion layer in order to suppress a reduction in the response speed of the photodiode, due to the accumulation of electronic charge in the vicinity of the depletion layer at the interface between the high resistivity P-type epitaxial layer 3 and the N-type epitaxial layer 4. Thereby, the response speed of the photodiode can be improved.
Typically, in a write mode in which data is written into the information recording side of an optical disk, writing of data, and reading of an address signal for verifying whether or not the written data is accurately recorded at a predetermined position on the information recording side, are alternately performed.
In the write mode of the optical disk apparatus, the writing of data is carried out by irradiating the information recording side of an optical disk with high-power laser light. In the mode of reading an address signal, the information recording side of an optical disk is irradiated with laser light having constant power much lower than the power of the laser light used for the writing of data. The laser light is reflected from the optical disk, and 2/100th or less of the reflected light in the write mode is detected by the photodiode.
The photodiode has to reliably detect photocarriers generated by the reflected light of a very small power of laser light when reading an address signal in the write mode. To satisfy such a requirement, the photocarriers generated by the reflected light of the high-power laser light are collected as photoelectric current at high speed when writing data, and an excess portion of the photocarriers (carriers having a long movement time) are subjected to recombination so as to prevent the excess photocarriers from contributing to the photoelectric current (signal). Thereby, subsequent reading of the address signal can be rapidly carried out.
However, the speed of writing data by an optical disk apparatus onto an optical disk is being further increased and the light power of laser light in writing data is also being increased. As the light amount of laser light is increased, the photodiode is likely to fail to read an address signal immediately after writing data onto the optical disk in the write mode.
The inventors have analyzed this phenomenon using simulation. As a result, it was found that when writing data onto an optical disk, a portion of photocarriers generated in a region of the semiconductor layer of the photodiode deeper than the PN junction region are moved and reach the PN junction region, and are detected as a photoelectric current, i.e., the movement time of the photocarrier is elongated to such a level. When the level of a signal (photoelectric current) generated by the photocarrier having a long movement time is greater than or equal to a predetermined level compared to the level of the address signal, the signal generated by the photocarrier having a long movement time is superposed with the address signal, so that a photoelectric current representing the address signal may not be read out.
Hereinafter, a mechanism underlying the case when the photoelectric current representing an address signal cannot be read out in the photodiode (FIGS. 13A and 13B) in the write mode, will be described.
FIG. 14 is a graph showing the response characteristics of the photodiode shown in FIGS. 13A and 13B with respect to pulsed laser light (wavelength: λ=780 nm). This graph shows a change over time in photoelectric current (output current) generated by taking, into an external circuit, photocarriers generated in each region within the photodiode by reflected light of the high-power pulsed laser light when writing data in the write mode of the optical disk apparatus.
The upper portion of the graph in FIG. 14 shows the semiconductor layers of FIG. 13B, where region B indicates the N-type epitaxial layer 4, region C indicates the high resistivity P-type epitaxial layer 3, region D indicates a slant region of the P+-type buried diffusion layer 2 nearer the high resistivity P-type epitaxial layer 3, region E indicates a region of the P+-type buried diffusion layer 2 in the vicinity of the peak of the impurity concentration and nearer the high resistivity P-type epitaxial layer 3, and region F indicates a region of the P+-type buried diffusion layer 2 from the vicinity of the peak of the impurity concentration to the low resistivity P-type semiconductor substrate 1.
A solid line indicated by A in FIG. 14 shows the response speed of overall photocarriers, indicating that photocarriers generated by the reflected light of high-power laser light are taken as a photoelectric current into an external circuit, and it takes about 14 nsec for the photoelectric current of the photocarriers to decrease by a factor of 2/100th or less which can be read out as an address signal.
The photocarriers generated in the regions within the photodiode due to the reflected light of the high-power pulsed laser light entering the photodiode can be divided into four components below:
(i) a photocarrier component which is generated in the depletion layer provided at the interface between the N-type epitaxial layer 4 and the high resistivity P-type epitaxial layer 3 and is moved at high speed due to the electric field in the depletion layer (photocarriers generated in region B and region C);
(ii) a photocarrier component which is generated outside the depletion layer at the interface between the N-type epitaxial layer 4 and the high resistivity P-type epitaxial layer 3 and is moved due to the internal electric field of the P+-type buried diffusion layer 2 (photocarrier in region D);
(iii) photocarrier component which is generated at the region in the vicinity of the peak of the impurity concentration in the P+-type buried diffusion layer 2 and is moved by diffusion due to the concentration difference (photocarriers generated in region E); and
(iv) a photocarrier component which is generated in a deeper region of the P+-type buried diffusion layer 2 between the region in the vicinity of the peak of the impurity concentration and the low resistivity P-type semiconductor substrate 7, and is substantially extinguished by recombination and slightly contributes to the photoelectric current (photocarriers generated in region F).
Among the photocarrier components (1) to (4), the photocarrier component (i) has the highest response speed, the photocarrier component (ii) has the second highest response speed, the photocarrier component (iii) has the third highest response speed. The photocarrier component (iv) is substantially extinguished by recombination and only slightly contributes to the photoelectric current, and has the lowest response speed.
The most of the photocarrier component (i) is taken as a photoelectric current to an external circuit within about 5 nsec. The most of the photocarrier component (ii) is taken as a photoelectric current to an external circuit within about 26 nsec. The most of the photocarrier component (iii) is taken as a photoelectric current to an external circuit within about 28 nsec. A relatively large portion of the photocarrier component (iv) is not taken as a photoelectric current to an external circuit even after 30 nsec.
FIG. 15 is a graph showing a change in light intensity from the surface of the photodiode in the depth direction when the reflected light of high-power pulsed laser light (wavelength: λ=780 nm) enters the photodiode shown in FIGS. 13A and 13B. The light intensity is exponentially decreased from the surface of the photodiode in the depth direction. For example, when the distance from the surface of the photodiode to the peak of the impurity concentration in the P+-type buried diffusion layer 2 is 17 μm, the light intensity in the vicinity of the interface between region E and region F is reduced to 13% of the light intensity at the surface of the photodiode.
The absolute numbers of photocarriers generated in the surface and the regions of the photodiode are proportional to the light intensity of the respective regions. Therefore, 80% or more of the total photocarriers are generated in regions B, C, and D.
The photocarrier components (iii) and (iv) are generated in regions E and F, respectively. Both regions E and F are deeper from the surface of the photodiode than the depletion layer, so that the absolute number of generated photocarriers is small. Therefore, in the data read mode of a typical optical disk apparatus, the photocarrier components (iii) and (iv)have substantially no influence on the level of an address signal when reading the address signal.
In contrast, in the data write mode of the optical disk apparatus, the amount of reflected light entering the photodiode from an optical disk is large in writing data, so that the absolute numbers of the photocarrier components (iii) generated in region E and (iv) generated in region F are increased.
When reading an address signal immediately after writing data, a light signal detected by the photodiode has a small power which is 2/100th or less of the reflected light from an optical disk when writing data. Therefore, when reading an address signal immediately after writing data, the photocarrier components (iii) generated in region E, and (iv) generated in region F, are increased in writing data. Therefore, the level of a photoelectric current generated by these photocarriers having a low response speed is greater than or equal to a predetermined level immediately after the writing of data as compared to the level of a photocarrier generated by an address signal, and the photoelectric current having a low response speed is superposed to the photoelectric current generated by the address signal, so that a photoelectric current representing the address signal may not be detected.
Such a phenomenon is likely to occur when writing of data is performed at high speed in the data write mode of an optical disk apparatus and pulsed laser light having a high power and a large amount is used. This is a large problem when a higher data write speed is desired.